Trust is defined as 'the firm belief in the reliability, truth, ability or strength of someone'. It does not define it as 'the firm belief that someone will do the right thing' or 'the firm belief that someone will do to what I want them to do'. Therefore, the first thing parents need to learn about trust in their relationship with their teenager is that while it is attached to our parental expectations, we have to be mindful that they are just teens.
For instance, if we expect our teen to be home on time and they have that good habit, we generally trust that they will be. And they most likely are. Great! But if our teen has been leaving their room a mess and we ask that they begin picking it up regularly, we expect that they will listen, but know that they are teenagers. They may need a little more incentive. So, we trust them to do what is asked but know they may do what they have always done - which is not pick up the room. If the room is picked up, we are happy. If not, we proceed to step two and add or follow through with a consequence. The key is to always remember that you should trust the fact that your child is a teen who won't always listen and will need your guidance instead of trusting your teen to do what is expected of them automatically. If you can do this, you will be able to keep your emotions calm when your teen does something wrong.
And yes, that goes for the big things as well. You can trust that your teen knows right from wrong and trust that your teen will do the right thing most of the time, but know that when your teen pushes their limits or experiments within the world around them that they will make big mistakes and betray your trust. They may drink or try drugs, bully someone, cheat on a test or have sex, even though they know - unequivocally - that these things are wrong. When caught they may even lie about it. Why? They don't honestly see that these behaviors are about the trust between the two of you until it's too late. So, after you deal with the situation at hand, always remember to help your teen regain your trust.
Which brings me to my next point, never use the trust you have in your teen or the fact that you at one time lost your trust against them. Refrain from saying things like, "I'm not sure I can trust you." or "I trusted you and you messed it all up." Instead, you can remind them that your are currently trusting them, you expect them to do the right thing. Using the above example, you could say, "I'm trusting that you will have your room clean when I get home from work today." This sends your teen a clear message of what they need to do and when without poking at past mistakes - which helps build or strengthen trust in your relationship.
Parents need to understand that trust is an essential part of their relationship with their teens and that their teenagers need their trust in order to grow. Without it, teens have a harder time building self-confidence, developing positive relationships and growing into a successful young adult. So, let your teen know that not only do you want to trust them but, they can trust you to be their parent. Tell them with your words and with your actions that you will sometimes make mistakes too, but they can trust you want the best for them and you will continue to do your best by them. (Hugs work well here.)